MACD (moving average convergence/divergence) is a technical analysis indicator created by Gerald Appel in the late 1970s. It is used to spot changes in the strength, direction, momentum, and duration of a trend in a stock’s price.
The MACD “oscillator” or “indicator” is a collection of three signals (or computed data-series), calculated from historical price data, most often the closing price. These three signal lines are: the MACD line, the signal line (or average line), and the difference (or divergence). The term “MACD” may be used to refer to the indicator as a whole, or specifically to the MACD line itself. The first line, called the “MACD line”, equals the difference between a “fast” (short period) exponential moving average (EMA), and a “slow” (longer period) EMA. The MACD line is charted over time, along with an EMA of the MACD line, termed the “signal line” or “average line”. The difference (or divergence) between the MACD line and the signal line is shown as a bar graph called the “histogram” time series (which should not be confused with the normal usage of histogram as an approximation of a probability distribution in statistics – the commonality is just in the visualization using a bar graph).
A fast EMA responds more quickly than a slow EMA to recent changes in a stock’s price. By comparing EMAs of different periods, the MACD line can indicate changes in the trend of a stock. By comparing that difference to an average, an analyst can detect subtle shifts in the stock’s trend.
Since the MACD is based on moving averages, it is inherently a lagging indicator. However, in this regard the MACD does not lag as much as a basic moving average crossing indicator, since the signal cross can be anticipated by noting the convergence far in advance of the actual crossing. As a metric of price trends, the MACD is less useful for stocks that are not trending (trading in a range) or are trading with erratic price action.